Generally, in a wavelength-division multiplexing communications system, each station (node) uses an optical add-drop multiplexer (OADM) having a function of adding and dropping each wavelength and a function of controlling gain wavelength characteristics, or uses an in-line amplifier (ILA) serving as an optical amplifier for relaying data between OADMs.
FIG. 1 illustrates an exemplary configuration of a conventional add-drop transmission device. This add-drop transmission device includes optical couplers 51, 54, and 61, a preamplifier 52, an optical demultiplexer 53, an optical switch 55, a variable optical attenuator (VOA) 56, a beam splitter 57, a photodetector 58, an optical multiplexer 59, a post amplifier 60, optical supervisory channel (OSC) transceivers 62 and 65, and transponders 63 and 64. The optical coupler 54, the optical switch 55, the VOA 56, the beam splitter 57, and the photodetector 58 are provided for each wavelength.
The optical signal of each wavelength is branched by the optical coupler 54, and is converted into an electric signal by the transponder 63. Also, an electric signal to be transmitted is converted into an optical signal of a corresponding wavelength by the transponders 64, and is added to the optical path by the optical switch 55.
As a method of compensating for variations in the optical power between wavelengths in an add-drop transmission device such as that described above, an optical signal level controlling method is suggested in which the VOA 56 in the device automatically controls such variations (see Patent Document 1 for example). In this method, the photodetector 58 detects the power level of each wavelength, and the amount attenuated by the VOA 56 is adjusted so that the power level is made to be uniform.
FIG. 2 illustrates an exemplary configuration of a conventional relaying device. This relaying device includes optical couplers 71 and 73, an in-line amplifier 72, and OSC transceivers 74 and 75.
Patent Document 1: Japanese Laid-open Patent Publication No. 2004-007058